Subpicosecond degenerate four-wave mixing and optical Kerr effect of organic dyes in excited states

Abstract

The third-order optical nonlinearities of electronically excited states were measured for three liquid scintillation dyes such as bis-MSB by means of subpicosecond degenerate four-wave mixing (DFWM) and optical Kerr effect (OKE) techniques. Molecular hyperpolarizability γ DFWM of the first excited singlet state of bis-MSB in tetrahydrofuran was estimated to be ca. 7×10 −56 A s m 4 V −3 at 780 nm. The enhancement factor of bis-MSB on UV excitation was as large as ca. 1.5×10 4. Temporal profiles of DFWM signal were measured with time resolution of 0.3 ps, and were found to consist of two components, the coherent instantaneous nonlinear response and the delayed response with decay time constants of ca. 9 ps. Though the profiles of OKE lacked or had very little coherent instantaneous component, delayed response had decay time constants almost same as those of DFWM. This delayed response can be explained in terms of molecular reorientation, the laser beam-induced reorientation of molecules in the direction of the laser beam polarization.